Magnetic disks such as hard disks and removable hard disks (e.g., floppy disks and MO disks) are now used as external storage for computers. A magnetic disk is a disk coated with a magnetic material on its surface, and data are written on or read from the disk using a magnetic head.
Commonly, a hard disk for computers comprises a hard aluminum substrate coated with a magnetic material on its surface. One or plural such hard disks are received in a closed casing, wherein they are rotated at high speed. A magnetic head is slightly floated from the surface of the hard disk for data write/read purposes.
Thus, the hard disk is usually designed to write and read information by means of a magnetic head located in the vicinity of the surface of the hard disk and floated thereover. Such a magnetic head is called a “floating head”. The distance between the magnetic head and the surface of the hard disk is very short, ordinarily of the order of a few μm or less. In addition, the hard disk is rotated at high speed; as the magnetic head comes in collision with the surface of the hard disk by reason of entrance of dust, impacts from outside, sudden power failures, etc., data may vanish or the magnetic head may break down. As a matter of course, when there are projections, if minute, from the surface of the hard disk, the magnetic head may collide therewith, causing errors. Although depending on the size and number of projections from the surface of the hard disk, the magnetic head may break down due to impacts on collision.
On the other hand, the magnetic head is mounted on the floating slider to write and read data on and from the hard disk while slightly floated over the surface of the hard disk. To reduce spacing losses of the magnetic head as much as possible upon writing and reading, the floating quantity of the magnetic head (the distance from the magnetic head to the surface of the hard disk) should now be reduced to the greatest extent practicable. In some systems known to date, the floating quantity is dwindled from a conventional several μm down to the order of 50 to 70 nm.
To eliminate minute projections on the surface of the hard disk, an aluminum substrate for hard disks must greatly be improved in terms of flatness and smoothness. For this reason, aluminum substrates are now fabricated by lathing or pressure annealing an aluminum substrate blank to remove the undulation of the principal surface, then forming an electroless plating layer on that principal surface, and finally polishing the plated layer to a mirror-smooth state.
However, such an aluminum substrate processing method is troublesome and poor in productivity, making it difficult to cut down on production costs. In addition, it is impossible to provide such an aluminum substrate with the projection-and-pit arrangement necessary for where servo marks are to be formed.
In recent years, it has thus been put forward to fabricate magnetic disk substrates such as hard disk substrates using thermoplastic resins. Thermoplastic resins can easily be configured into moldings of any desired shape by melt processing such as injection molding. In addition, if the surface roughness of molds or stampers used for injection molding is previously reduced, it is then possible to obtain moldings excelling in smoothness.
Even by use of injection molding of thermoplastic resins, however, it is in effect very difficult to obtain magnetic disk substrates having high reliability.
JP-A 04-170425 proposes forming a disk substrate with a molding material that is a hydrogenated product of a ring-opening polymer of a norbornene monomer, wherein the volatile component content is 0.3% by weight or lower and the content of foreign matter of 0.5 μm or greater is 1×105 or less. This disk substrate is reduced in terms of voids and silver streaks resulting from volatile components as well as the content of foreign matter, and so is useful for an optical disk substrate. Voids and silver streaks are responsible for errors upon reading of signals by an optical disk using laser beams; however, molding defects such as voids and silver streaks can be eliminated by decreasing the content of volatile components, thereby correcting the optical disk for possible errors.
However, the results of investigations by the inventors have taught that although the disk substrate set forth in the aforesaid publication is useful for an optical disk substrate, a reliability problem arises when it is used for the substrate of a magnetic disk that operates proximately to a magnetic head as is the case with a hard disk.
Possible reasons could be that (1) no full removal of low-molecular components is achievable under drying conditions that the hydrogenated products of ring-opening polymers specified in the publication are vacuum dried at 260° C., and (2) at the foreign matter content level of 6×104 to 9×104/g described in the publication, any complete prevention of minute projections from the surface of the disk substrate is unachievable.
WO 98/08217 proposes a disk substrate comprising a resinous disk substrate, characterized in that on the surface to be scanned by a floating head there is no projection having a height of 50 nm or greater. This publication also diskloses a disk substrate fabrication method by injection molding, wherein a resin solution is filtered by a filter to prepare a resin in which the content of particles of 0.5 μm or greater in particle diameter is 1×104/g or less, and the resin is injection molded to a disk substrate.
The magnetic disk manufactured using the disk substrate set forth in the aforesaid publication, because of having no minute projection on the surface to be scanned by a floating head, is found to be much more reduced in terms of errors upon writing and reading of information than conventional disks. However, the results of studies by the inventors have revealed that the effect of this magnetic disk on prevention of errors is still less than satisfactory, and that adhesion between the disk substrate and the magnetic layer becomes insufficient.